Natural gas is an important resource widely used as energy source or as industrial feedstock used in, for example, manufacture of plastics. Comprising primarily of methane, natural gas is a mixture of naturally occurring hydrocarbon gases and is typically found in deep underground natural rock formations or other hydrocarbon reservoirs. Other components of natural gas include, but are not limited to, ethane, propane, carbon dioxide, nitrogen, and hydrogen sulfide.
Typically, natural gas is transported from source to consumers through pipelines that physically connect a reservoir to a market. Because natural gas is sometimes found in remote areas devoid of necessary infrastructure (e.g., pipelines), alternative methods for transporting natural gas are needed. This situation commonly arises when the source of natural gas and the market are separated by great distances such as a large body of water. Bringing this natural gas from remote areas to market can have significant commercial value if the cost of transporting natural gas is minimized.
One alternative method of transporting natural gas involves converting natural gas into a liquefied form through a liquefaction process. Because natural gas is gaseous under standard atmospheric conditions, it must be subjected to certain thermodynamic processes in order to be liquefied. In its liquefied form, natural gas has a specific volume that is significantly lower than its specific volume in its gaseous form. Thus, the liquefaction process greatly increases the ease of transporting and storing natural gas, particularly in cases where pipelines are not available. For example, ocean liners carrying LNG tanks can effectively link a natural gas source with a distant market when the source and market are separated by large bodies of water.
Converting natural gas to its liquefied form can have other economic benefits. For example, storing LNG can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be more easily “stockpiled” for later use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
In some instances, natural gas streams can contain relatively high concentrations of nitrogen. Nitrogen is inert and lowers the energy value per volume of natural gas. Thus, specification for natural gas pipelines typically limit the concentration of nitrogen. Limiting the concentration of nitrogen may also be important during liquefaction of natural gas. High nitrogen concentrations in natural gas that is subjected to liquefaction in a LNG facility can present one or more of the following drawbacks: (1) the natural gas can be more difficult to condense; (2) the heating value of the natural gas used as fuel gas for the LNG facility's gas turbines can be greatly diminished; and (3) LNG produced by the facility may be out of spec.
Some LNG facilities employ nitrogen removal units (NRUs) to lower the concentration of nitrogen in the natural gas stream to an acceptable level. Some conventional nitrogen rejection units (NRU) integrated within an LNG facility may be arranged in a two or three column configuration. Liquefaction processes using conventional three column NRU often relies solely on auto-refrigeration. Some of the problems associated with conventional three column NRU processes include, but are not limited to, slow startup and unstable operation due to variable feed compositions. These NRU columns are usually non-refluxed stripped or reboiled absorbers and can produce a nitrogen vent stream. Another drawback of some conventional nitrogen rejection units is that the resulting nitrogen vent stream can contain relatively high levels of methane. Some conventional nitrogen rejection units produce nitrogen vent streams containing between about 1% to about 1.5% methane (by mole %).